Pure chromium



Dec. 9, 1958 R. s. DEAN ETAL PURE CHROMIUM Filed March 27, 1957INVENTORS REGINALD 5: 05AM ITQANK X NCANLEY ATTORNEY PURE CHROMIUMReginald S. Dean, Hyattsville, and Frank X. McCaWley, Cheverly, Md.,assignors to Chicago Development Corporation, Riverdale, Md., acorporation of Delaware Application March 27,1957, Serial No. 648,888

1 Claim. (Cl. 75- 176) This invention relates to pure, chromium,articles made from it and methods for the preparation of same. It hasfor its aim the production of chromium by' electrorefining in the formof idiomorphic crystals and crystal intergrowths of hexagonal form, andthe production from such chromium of alloys of chromium with othermetals. It also has for its aim, the production of pure chromium in theform of ductile adherent coatings on metals.

In the known art, chromium has been produced by a variety of methods.Chromium made, for example, by electrolysis of aqueous solution and bymagnesium reduction of chromium chloride contains oxygen which must beeliminated by other procedures as stated by Sully, Chromium, p. 55,Batterworths, London, p. 1954. The same authority states that chromiumobtained from amalgam is very finely divided.

Pure chromium has been produced by dissociation of the iodide in theform of a coating on a wire. Crystals are not idiomorphic ormacroscopically distinct.

Reduction of CrCl with hydrogen producedchromium powder of only 99%purity according to Maier in U. S. Bureau of Mines, Bull. 436,109(1942,).

Electrolysis of fused salts has been disclosed by F. Krupp in U. K.Patent 197,887 (1922); Kroll et alt, U; S. Bureau of Mines Rep. 4752,December 1950, found, that metallic chromium produced in this Waycontained 1.6% oxide.

A hexagonal form of chromium has been noted in plates but not inidiomorphic crystals. Further, the hexagonal structure is converted tothe usual body centered cubic form by heating one hour at 150 accordingto Snavely, Trans-Electro Chem. Soc. 92,537 (1947) In one embodiment ofour present invention, we produce chromium as macroscopic idiomorphiccrystals of hexagonal form containing less than 01% oxygen; a group ofsuch crystals is shown in Figure 1 magnified 2X; the hexagonal form isclearly shown by the outline of the crystals. The atomic arrangement ofthese crystals is shown by X-ray spectrometry to be body centered cubic;

the crystals are slowly attacked by dilute sulphuric and hydrochloricacid but are readily passivated by dilute nitric acid.

The chromium content of the crystals is more than 99.99% Cr. peratureand become highly ductile at slightly elevated temperatures.

In a preferred process for the production of these chromium crystals,which we will describe in detail in examples, we pass a direct currentfrom a comminuted crude chromium anode to an iron cathode in anelectrolyte of sodium chloride having dissolved therein chromiumchlorides and metallic sodium. The first step of such a process is theprecipitation of small chromium crystals in a salt layer at the surfaceof the cathode. These crystals consolidate into a layer of consolidatedchromium forming a ductile, highly protective'plate on the metalcathode. The large idiomorphic crystals and They are somewhat ductile atroom temcrystal intergrowths of our invention are attached to thecathode by the salt layer containing dispersed fine chromium crystals.

The production of this plate is one of the objects of our invention, andthe plated objects are one of the articles of our invention.

The plate consists of micro-crystals and is highly deformable andductile. This layer of consolidated metal has an electrical resistanceof 3.6 microhms cm. It is highly protective to-steel in salt water.

The chromium of our invention is particularly desirable for formingalloys with hexagonal metals such as titanium, zirconium, and cobalt bythe processes of powder metallurgy.

Alloys made in this way are free from objectionable characteristicsassociated with chromium alloys of the known art. 1

The chromium crystals of our invention can also be used for theproduction of alloys by arc melting of consumable electrodes containingcrystals of chromium of our invention and other high purity metals suchas crystal intergrowths of pure titanium and zirconium.

Having now described our invention in its general terms, we willillustrate it by example.

We comminute this material to pass an 8 mesh screen, and place it in aforaminous nickel basket concentrically disposed around a steel rodin-an electrolytic cell provided with an argon atmosphere and having anelectrolyte of molten NaCl. in which there is dissolved 5% Cr aschromium chloride, average valence 2.05 and .1% metallicsodium.

We prepare this electrolyte by melting a mixture of CrCl and NaCl,placing it in the cell described and passing a direct current fromchromium in. the anode basket to the steel cell wall at 5 amperes until1.2 faradays of current have been passed for each 52 grams of chromiumpresent. We then place ferrochrome in the basket.

The approximate surface of the ferrochrorne in the anode basket is sq.ft. and the immersed surface of the initial cathode rod is .5 sq. ft.

We pass a direct current of 100 amperes for 3 hours at 800 C. 4

The cathode rod with adhering material is then removed from the bath andcooled, without access to air, 290 grams of macroscopic chromiumcrystals having a hexagonal form are removed from the cathode and washedwith 1% HCl to remove salt. After drying these crystals had a hardnessof 126 D. P. H. and were cold ductile.

They had less than .0l% oxygen and no other detectable impurities. Thecomposition of the bath was not changed by the passage of current.

The cathode rod from which the crystals were removed was coated with alayer of salt 10 mils thick containing dispersed fine chromium crystals.This layer was scrubbed off and the rod was found to be plated with alayer 3 mils thick of ductile chromium in a continuous nonporous plateprotective to steel in salt water.

The electrical resistance of the ductile chromium layer was 3.6 microhmscm. at room temperature.

Both the crystals and the plate show on analysis less than .0l% oxygenand no other determinable impurities.

3 Example 11 In this example, we proceed as in Example I, except asfollows, the electrolyte is 65% SrCl 35% NaCl in which is dissolved 8.9%Cr as chloride having an average valence of 2.16 and .6% dissolvedalkalinous metal; the temperature was 600 C.; the cathode is an alumiumrod.

The results after 3 hours passage of current are identical with those ofExample I including the formation on the aluminum rod of a non-porousductile plate.

Example 111 In this example, We take crystals of pure chromium preparedin accordance with Example I and mix them with pure comminutedelectrolytic cobalt in the proportion of 65% by weight, cobalt 35% byweight chromium. We compact this mixture at 50 tons per square inchpressure in a steel die. We then heat the so-formed compact to 950 C. ina vacuum of .01 micron for 8 hours. Th resulting alloy when quenchedfrom 950 C. is entirely the hexagonal beta phase of the chromium cobaltsystem as described by Elsea, Westerman and Manning, Trans. A. I. M. E.,180,579 (1949). The alloy is malleable at room temperature and has ahardness of 300 Brinell. The quenched alloy can be hardened by heatingto 600 C. After 3 hours at this temperature, it has a hardness of 500Brinell without brittleness.

Example I V In this example, we take titanium crystal intergrowthshaving an oxygen content of less than .01% and mix these with 9% ofchromium crystals produced in accordance with Example I. We compactthese crystals into a consumable electrode and melt the electrode in anarc furnace having a water-cooled hearth at an argon pressure of .1micron. The resulting ingot is heated in pure argon to 1000 C. andrapidly cooled.

The resulting beta titanium alloy is ductile and has a hardness of 250Brinell. On heating to 600 C. for 1 hour, it is hardened to 500 Brinellwithout becoming brittle.

Example V In this example, we take crude chromium in pulverulent formand mix it with CrCl in proportions to form TiCl and add this to NaCl insuch proportion that the total Cr content is 10% by weight. We heat thismixture to 800 C. and place it in a cell as described in Example I witha comminuted chromium anode and pass a current of amperes for 3 hours.The resulting bath analyzes 4 9.8 total soluble chromium; averagevalence determined by ferric sulphate solution 2.03; sodium by hydrogenevolution in acidified ferric sulphate solution 0.5%.

We then place a molybdenum cathode in the cell having an immersedsurface of 2 square feet. The chromium in the anode basket has a surfaceof 1000 square feet. We pass a current of 500 amperes for 3 hours at 800C.

The molybdenum cathode is then removed from the bath and cooled inargon. 1410 grams of macroscopic hexagonal chromium crystals are removedfrom the cathode.

The crude chromium in the anode analyzed 1.6% oxygen, 3% iron, balancesubstantially chromium. The crystals obtained from the cathode afterwashing analyzed less than .01% O and less than .01% Fe with nodeterminable amount of other impurities.

What is claimed is:

As an article of manufacture, macroscopic idiomorphic crystalintergrowths of chromium hexagonal as formed at above 750 C. andcontaining as formed less than 0.01% O and substantially no otherimpurity, said crystal intergrowths being pseudomorphs after thehexagonal form but having the crystal structure of alpha chromium whencooled to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,163,354 Schmidt et a1 June 20, 1939 2,446,996 Bouton et al Aug. 17,1948 2,631,936 Kuhlmann Mar. 17, 1953 2,656,269 Dunn et al Oct. 20, 19532,658,266 Du Rose Nov. 10, 1953 2,702,239 Gilbert et a1 Feb. 15, 19552,752,303 Cooper June 26, 1956 2,780,545 Blank et a1 Feb. 5, 19572,782,114 Preston Feb. 19, 1957 2,789,943 Kittelberger Apr. 23, 1957OTHER REFERENCES Institute of Metals, Journal, vol. 80, 1951-52; pages112-114.

Institute of Metals, Journal, vol. 83, 1954-; pages 121-125.

Methalloberllache, 1953 (A)7, (10), -148. Abstracted on page 347,Institute of Metals Metallurgical Abstracts, vol. 22, Sept. 1954-Aug.1955. Metallographic Investigation on Electrodeposited Chromium, Koch eta1.

